<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<style>
		#msg {
			display: none;
			background-color: #f00;
		}
	</style>
</head>
<body>

	<label>输入框：</label>

	<input id="ipt" type="text">

	<p id="msg">输入字符必须以字母开头！</p>

	<script>
		
	var ipt = document.getElementById('ipt');
	var msg = document.getElementById('msg');
	ipt.onblur = function() {
		var val = this.value;

		if (isStartWithString2(val)) {
			msg.style.display = 'none';
		} else {
			msg.style.display = 'block';
		}
	};

	function isStartWithString(str) {
		var reg = /^([a-zA-Z].+)$/;
		return reg.test(str);
	}

	function isStartWithString2(str) {
		var fchar = str.charAt(0),
			charCode = fchar.charCodeAt();

		if (charCode >= 65 && charCode <= 122) {

			if (charCode > 90 && charCode < 97) {
				return false;
			}

			return true;

		}

		return false;
	}

	</script>
	
</body>
</html>